The modulatory effect of amiloride on glycine-activated current (I-Gly) was investigated in acutely dissociated rat spinal dorsal horn neurons using the whole-cell patch clamp technique. Amiloride inhibited I-Gly reversibly in a concentration-dependent manner. It shifted the concentration-response relationship to the right without altering the maximum response and Hill coefficient of the I-Gly. Amiloride did not change the ion selectivity of glycine receptor either. In addition, Na+- or Ca2+-free extracellular solutions and intracellular application of amiloride did not alter the amiloride inhibition of I-Gly. These results indicate that amiloride directly inhibited the glycine receptor response by decreasing the affinity of glycine to its receptor. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.